This invention relates to a process for the treatment of a gallium compound-containing waste for the recovery of the gallium value as metallic gallium.
Gallium is now increasingly consumed in a variety of fields especially in manufacture of semiconductors. Since gallium is not abundantly available in nature, the recovery of gallium from gallium-containing wastes becomes now more and more important.
In the manufacture of semiconductor wafers of gallium arsenide, gallium phosphide, or the like gallium compound, a large amount of powder or fine particles of such gallium compounds are produced during, for example, cutting single crystals to right cylinders and cutting of the cylinders into wafers. Since these cutting operations are desirably carried out with the use of a metal-working fluid such as a cutting oil or an organic compound-containing machining water, the gallium-containing waste produced by such operations is in the form of a sludge, generally called "dross". Such an oil or organic compound deposits on the surface of the gallium compound particles and cannot be removed by washing even with an organic solvent such as quinoline. Thus, it is very difficult to recover the gallium value from such sludge-like wastes.
A lot of methods for recovering gallium from gallium compound-containing wastes are known. Among others, a method involving vacuum thermal decomposition of gallium compounds is known to be effective in obtaining metallic gallium from gallium-containing wastes resulting from the manufacture of semiconductors (U.S. Pat. No. 4,362,560).
The vacuum thermal decomposition technique, however, cannot be applied for the treatment of the above-mentioned sludge-like wastes because the metal-working fluid foams violently during the vaccuum thermal decomposition. By distilling the sludge-like waste under vacuum with or without stirring, there is obtainable dried powder of the gallium compound. Such dried powder, however, cannot be effectively subjected to vacuum thermal decomposition since the fine powder or dust is easily scattered and flies off from the decomposition reactor. Moreover, the dried powder is not effectively thermally decomposable even the temperature is raised sufficiently.